Molybdenum dioxide

Water solubility

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

water solubility

Type of information:

experimental study

Adequacy of study:

weight of evidence

Study period:

2008-01-29 to 2008-04-17

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

guideline study with acceptable restrictions

Remarks:

The concept of saturation water solubility, which forms the basis for this guideline test, is not directly applicable to molybdenum dioxide. Please refer to the discussion in the endpoint summary.

Qualifier:

according to guideline

Guideline:

OECD Guideline 105 (Water Solubility)

Deviations:

yes

Remarks:

The flask shake method was used even though the water solubility of molybdenum dioxide was below 0.01 g/L.

GLP compliance:

yes (incl. QA statement)

Type of method:

flask method

Water solubility:

ca. 1.1 mg/L

Temp.:

20 °C

pH:

5

Details on results:

The results of the preliminary test was about 21 times higher than the results of the 1st main test, so the main test was repeated to confirm the result of the first main test (see attached material).

Conclusions:

Interpretation of results: slightly soluble (0.1-100 mg/L)
Under the conditions of this test (test duration up to 192 hours, see attached material), the mean water solubility of the test item molybdenum dioxide in bidistilled water (being in equilibrium with atmospheric carbon dioxide) at 20 °C is determined to be ca. 1.1 mg/L.
The mean pH of the saturated solutions was ca. pH 5.
In this study, 8 mg of the test item MoO2 were suspended in 50 mL of distilled water. Please refer to the discussion in the endpoint summary.

Reference 2

Endpoint:

water solubility

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

guideline study with acceptable restrictions

Remarks:

Tthe concept of saturation water solubility, which forms the basis for this guideline test, is not directly applicable to molybdenum dioxide. Please refer to the discussion in the endpoint summary.

Qualifier:

according to guideline

Guideline:

OECD Guideline 105 (Water Solubility)

Version / remarks:

adopted 27 July 1995

Deviations:

no

GLP compliance:

yes (incl. QA statement)

Type of method:

flask method

Water solubility:

ca. 23.3 mg/L

Temp.:

20 °C

pH:

3.5

Details on results:

ICP-OES measurement and determination of water solubility
Variations of up to 25 % are observed between samples taken at different time (some are above the limit of 15 % allowed by the test guideline). No tendency to increasing values with increasing equilibration times was observed. After 192 hours the day-today variation between two samples was 6.5 %. In consequence, the equilibration times used are long enough according to the test guideline.
For the determination of the water solubility the mean Mo concentration from 168 and 192 hours is calculated: Mean molybdenum concentration (n=4) = 17.5 +/- 0.8 mg/L
The water solubility of molybdenum dioxide is calculated as follows: (17.5 +/- 0.8) mg/L * 127.94 g/mol (MoO2) / 95.94 g/mol (Mo) = 23.3 +/- 1.1 mg/L
The water temperature had a standard deviation of +/- 0.5.
Observations:
During the study a dark grey residue has been observed. After filtration of the solution with a syringe filter (porosity 0.45 µm) the solution was clear and without colour. The temperature of the water bath was held at 20.0 +/- 0. 5°C. No further observations have been made.

Conclusions:

Interpretation of results: slightly soluble (0.1-100 mg/L)
Under the conditions of this test, the water solubility of molybdenum dioxide in distilled water at 20.0 +/- 0.5 °C after 8 days is determined at 23.3 +/- 1.1 mg/L.

Description of key information

Under laboratory conditions, molybdenum dioxide powder reacted slowly with water. It is assumed that a thin layer of molybdenum oxide  initially reacted with the water, resulting in the release of molybdate ions (MoO4 2-) into solution. After separating the solution from the residual particles, the analysed concentrations of dissolved molybdenum were in the order of magnitude of approximately 1 -20 mg/L after a test period of ca. 1 week, depending on the loading.

In addition to the standard OECD TG 105 water solubility study, the substance has also been tested in a bioelution study (in vitro bioaccessibility) in artificial gastric juice. This test and the results are described in the stand-alone document „MoCon read-across concept (human health)“, which is attached to the technical dossier in IUCLID section 13.

 

Additional information

The solubility of a substance in water is generally defined as the saturation mass concentration of the substance in water at a given temperature. The solubility in water is specified in units of mass per volume of solution (ECHA Guidance on information requirements and chemical safety assessment, Chapter R.7a: Endpoint specific guidance).

 

However, as also discussed in the ECHA guidance, this concept of saturation water solubility is not directly applicable to poorly soluble, inorganic or metallic compounds.

 

Solubility of a metal oxide depends on a variety of parameters: the chemistry and composition of the compound, the particle size, the surface area in contact with the solution, the amount of substance per solution volume (“loading”) and last but not least the characteristics of the aqueous medium (environmental or physiological media). Many oxides are generally considered “insoluble”. Nevertheless, oxides may react with water or a dilute aqueous electrolyte to form soluble or sparingly soluble cationic or anionic products. In this context, reaction or dissolution kinetics play a key role.

 

Two water solubility studies in accordance with OECD guideline 105 have been conducted with samples of molybdenum dioxide powder (Bär, 2009 and Rüdel, 2009). Because of the multitude of parameters influencing the water solubility of molybdenum dioxide, a definitive result for water solubility could not be derived.

In the study by Baer, 2009, molybdenum dioxide powder was suspended in water at a loading of 8 mg per 50 mL (i.e. 160mg/L). After the test period of ca. 1 week, a concentration of dissolved molybdenum of ca. 0.85 mg/L was analysed, corresponding to a "solubility" of MoO2 of ca. 1.1 mg/L.

In contrast, in the study by Ruedel, 2009, molybdenum dioxide powder was suspended in water at a loading of 1 g per 500mL (i.e. 2000 mg/L). After the test period of ca. 1 week, a concentration of dissolved molybdenum of ca. 17.5 mg/L was analysed, corresponding to a "solubility" of MoO2 of ca. 23.3 mg/L.

Clearly, the resulting concentration of dissolved molybdenum depends on the ratio of sample mass to solution volume.

Based on a weight of evidence approach and considering both studies, the following may be derived as a tentative result: Under the laboratory conditions described in the studies, molybdenum dioxide powder reacted slowly with water. It is assumed that a thin layer of molybdenum oxide initially reacted with the water, resulting in the release of molybdate ions (MoO4 2-) into solution. After separating the solution from the residual particles, the analysed concentrations of dissolved molybdenum were in the order of magnitude of approximately 1 -20 mg/L after a test period of ca. 1 week, depending on the loading.

In addition to the standard OECD TG 105 water solubility study, the substance has also been tested in a bioelution study (in vitro bioaccessibility) in artificial gastric juice. This test and the results are described in the stand-alone document „MoCon read-across concept (human health)“, which is attached to the technical dossier in IUCLID section 13.

Finally, it has been shown that molybdenum, even in the form of highly soluble salts, is of very low toxicity toward environmental organisms. Also any significant environmental exposure with molybdenum dioxide is quite unrealistic. Therefore, further tests regarding the dissolution of molybdenum dioxide (e.g. the transformation dissolution test) have not been conducted.